The present invention relates to a composition for a treatment agent for polyester fiber (polyester-fiber-treatment agent or PFTA). More specifically, the present invention relates to a PFTA that is capable of imparting smoothness and flexibility to polyester fiber at the same time that it can equip polyester fiber with nonflammability. Polyester (PE) fiber has better heat-settability, crease resistance, and dyeability than other synthetic fibers such as nylon fiber, acrylonitrile fiber, polyvinyl chloride fiber, and polypropylene fiber, and has therefore been used for clothing fabric where such properties are critical. However, polyester fiber suffers from poor flexibility and poor smoothness. In response to these problems, there have been attempts at improving the smoothness and flexibility of polyester fiber or PE fiber-based fabric through the application of various types of organopolysiloxanes, for example, dimethylpolysiloxanes, amino-containing organopolysiloxanes, epoxy-containing organopolysiloxanes, and carboxyl-containing organopolysiloxanes. However, all of the above-listed organopolysiloxanes are flammable compounds, and the flammability of PE fiber coated with them is therefore increased. In order to resolve this problem, various types of flame retardants have been blended into PE resins and the aforementioned polyorganosiloxane has then been applied to the PE fiber or fabric made from this flame-retarded PE resin. However, these methods are unsatisfactory because they cause a deterioration in the physical properties of the PE fiber.